The above discussion about ellipticals with shells is not complete. The
fate of the gas contained in the infalling disk needs to be considered
too. A typical disk galaxy of mass 1010M brings
along 108.5-9M of
neutral gas, which has to go somewhere. Only
Centaurus A
(76)
and NGC 2865 (cf.
77)
have detectable amounts of H
I. It seems easy to say that the gas will be heated up toward the
temperatures of the X-ray-emitting gas known to exist in field
ellipticals and S0s
(68),
e.g. in NGC 3923, or that it could form stars,
but what then about the presence of dust and gas in ellipticals?

To get a feeling for the statistics first, we have constructed a sample
of field ellipticals based on a catalog of E and S0 galaxies south of
-33°, thought to be complete down to mB = 13.8
(94,
95).
We reclassified all the galaxies in that catalog with radial velocities
less than 4000 km s-1; ellipticals were distinguished from
S0s by using
both the ESO and SRC films, which resulted in a list of 78 ellipticals
and 47 S0s complete down to mB = - 20.5 (H = 50
km s-1 Mpc-1). We estimate that only up to 5
galaxies in this list could be misclassified.

We then compared this list with that of Malin & Carter
(77) for
ellipticals with shells and with the lists of ellipticals with dust
(52,
120,
137,
155,
and our own notes). Among the 78 ellipticals are 10 with
shells, 9 with dust (of which 3 have shells as well), and 19 radio
detections as listed in
(96).
Among the 47 S0s there is 1 with a shell,
4 with clear dust lanes always aligned with the major axis, and 5 radio
detections. Of the 78 ellipticals 27 occur in clusters or very rich
groups. Since neither shell galaxies nor dust-lane galaxies occur among
these, we have a total of 51 ellipticals in a:representative field
sample. Thus shells or dust is observed in about 20% of the field
ellipticals, but there is surprisingly little occurrence of both
phenomena together. Perhaps this is partly exaggerated; for example,
if ellipticals are oblate and have shells of small azimuthal extent, the
projection angles most favorable for detecting dust lanes may not be so
favorable for detecting shells as well.

Slightly higher frequencies of occurrence of both phenomena have been
reported recently using higher resolution data. Schweizer
(115) finds on
the basis of 4-m plates that 16 out of 36, field ellipticals have shells
(ripples) and 9 out of 36 have dust. The occurrence of small dust lanes
near the center in several more galaxies
(97)
may increase the frequency
of dust in ellipticals to as high as 40%, comparable with the estimate
of the detection rate of ionized gas with equivalent width at
3727 ([O II]) larger
than 1 Å
(21).

With respect to the radio detections, 9 occur among the 27 cluster or
rich-group galaxies and 10 among the field ellipticals. Of the field
elliptical detections, 2 occur in ellipticals with both shells and dust
(For A and Cen A), 1 in a shell galaxy with no dust, and 4 in
galaxies
with dust and no shells. Thus the presence of dust increases the
detection rate from 10% to 60%, while the presence of shells does not
seem to affect the detection of radio emission. As far as ionized gas is
concerned, 19 of the 78 galaxies have been included in Caldwell's
(22)
list. Eleven of these have been detected: 6 out of 6 dust-lane galaxies,
and 5 out of 13 (40%) others. Thus the active galaxy phenomenon in
ellipticals seems to be associated with dust and gas but not with shells
or ripples. This supports indirectly the view that shells and ripples
are stellar phenomena.

The results from the orbit calculations and from the analysis of
preferred planes can also be applied to the ellipticals with
dust. Unlike S0s, ellipticals
do not have a well-defined main plane, and an ambiguity exists between
near-oblate systems tumbling around the short axis and near-prolate
systems tumbling around the long axis. This can only be settled if the
kinematics of both stars and gas is known and if an assumption is made
about the sense of tumbling with respect to the stellar streaming
motion. Another difference with the polar ring situation is that the
dust lanes occur mostly inside the optical image. In the case of Cen A,
the main body is most likely prolate on account of the sharply defined
shells, and then the warped dust lane can be in a stable configuration
if the figure tumbles retrograde with respect to the gas streaming
motions in the outer parts
(142).

For several ellipticals with dust, the gas rotates around the major
axis, while the stellar rotation, if present, is around the minor axis
(e.g. 11,
21,
22,
30,
82,
119).
In others, the two angular momentum vectors are more aligned
(22,
119).
In some cases the dust lane is really a thick annulus
(22,
51)
that is warped in its outer parts. Several dust
lanes exist that are not aligned with any principal axis
(51).
The case of a gas disk rotating around the same axis as the stars, but
with an antiparallel-spin vector
(12,
23),
strongly suggests an extragalactic
origin for the gas. Neutral hydrogen in some ellipticals (see
61)
apparently rotates in planes different from both the ionized gas and the
stars, e.g. in NGC 4278
(30,
89,
108).
This could indicate precession of a gas disk resulting from capture
(48).
Thus, if the gas is indeed of
external origin, the swallowing of dwarfs again seems a popular
candidate to explain its origin. The gas and dust should settle
eventually into a preferred plane, but the fate of the dwarf's stellar
component is now unaccounted for. So either the recipient ellipticals
are different from those that have shells, or the dwarfs are of a
different nature, or the collision parameters are different.